1. Field of the Invention
The present invention relates to a method and apparatus for paper feeding, and more particularly to a method and apparatus for paper feeding that is capable of handling multiple paper cassettes.
2. Discussion of the Background
Sheet separation has been an important function for a paper feed apparatus used in image forming apparatuses such as copying machines, printers, facsimile machines, and so forth. A sheet separation mechanism that separates a sheet from multiple sheets with a forward feed roller pair (feed roller) for sending a recording sheet forward and a reverse feed roller (separation roller) for reversing extra-transferred sheets using a torque limiter is particularly referred to as a forward/reverse roller system.
For example, in an image forming apparatus with a paper feed apparatus which adopts the forward/reverse roller system, stacks of recording sheets in a paper cassette or the like are sent by a pick-up roller sheet by sheet from an uppermost-positioned sheet of the stacks to a nip portion between the feed and separation rollers. At this time, one or more additional recording sheets may be erroneously transferred together with the uppermost-positioned sheet. This occurrence is referred to as a multiple sheet feed error.
At such a multiple sheet feed error, the paper feed apparatus based on the forward/reverse roller system is configured to separate the uppermost-positioned sheet from the additional recording sheets and to transfer only the uppermost-positioned sheet to an image forming section of the image forming apparatus. That is, the separation roller is configured to be kept in contact with the feed roller under pressure and is provided with a rotative force in a reverse direction relative to a transfer direction of the recording sheet. The separation roller may rotate in the reverse direction with a torque of at least 1 to 1.5 kgf-cm (kilogram force centimeter) via a torque limiter.
On the other hand, an image forming apparatus which includes a plurality of paper cassettes, sheet trays, or the like, has been provided in recent years in order to cope with increasing demand for being able to utilize various recording sheets of different sizes and kinds. In a paper feed apparatus of such an image forming apparatus, a plurality of paper feed mechanisms are provided so that the recording sheets in each paper cassette can be handled individually. Each paper feed mechanism of the paper feed apparatus generally includes a sheet elevation mechanism for lifting the stacks of recording sheets upwards to a predetermined paper feed position. For example, Japanese Laid-Open Patent Publication JPAP9-086680(1997) describes a sheet elevation mechanism. This mechanism is installed in each paper feed mechanism and lifts up a respective stack of recording sheets by causing a base plate in a paper cassette to pivot about a support shaft of the base plate.
Also, the above-described sheet separation mechanism included in each paper feed mechanism of the paper feed apparatus is generally configured to share a drive motor with all of the other sheet separation mechanisms from a cost reduction standpoint. Accordingly, the drive motor needs to bear a heavy load and, therefore, a reduction of a load required by each sheet separation mechanism becomes crucially important. In this aspect, the forward/reverse roller system generally reduces loads of the sheet separation mechanisms by disengaging the feed roller from the separation roller at the sheet separation mechanisms of the paper feed mechanisms which are in an inoperative mode. This technique is described in Japanese Laid-Open Patent Publication No. JPAP6-183600.
However, this technique has a problem in that the feed rollers of the sheet separation mechanisms of the paper feed mechanisms which are in an inoperative mode become free from contact with their respective separation rollers and tend to start to rotate by themselves due to a frictional rotation of a clutch mechanism. As a result, a recording sheet may be transferred forward by the rotating feed roller if the recording sheet is positioned near the feed roller. That is, the sheet separation mechanisms of the paper feed mechanisms which are in an inoperative mode may transfer recording sheets, which may cause a paper jam in the image forming section of the image forming apparatus.
The above-described problem may be avoided by adding ball bearings inside the clutch, coating a sliding surface inside the clutch with a relatively low-friction substance, or the like. However, these countermeasures increase a machine cost.